Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2002-08-19
2004-10-05
Flynn, Nathan J. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S466000, C257S452000, C257S465000, C257S199000, C257S186000
Reexamination Certificate
active
06800914
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor photodetector device and a manufacturing technique thereof, and particularly to a technique effectively applied to an improvement in reliability of mesa-type photodetector devices employing compound semiconductors.
Japanese Patent Laid-Open No. 2001-177143 discloses a structure in which a mesa is formed on a crystal to be a substratum as a photodetector employing compound semiconductors and a periphery of the mesa is embedded by a crystal made of a suitable material and having a proper carrier density (hereinafter, this structure will be referred to as “embedded mesa-type”).
FIG. 21
is a cross-sectional view illustrating an embedded mesa-type semiconductor photodetector described in the above-mentioned document. According to a brief description of a manufacturing method of this semiconductor photodetector, at first, a buffer layer
402
made of an n-type InAlAs crystal; a multiplication layer
403
made of an n-type InAlAs crystal; a field control layer
404
formed of a laminated body of a p-type InAlAs crystal and a p-type InGaAs crystal; an absorption layer
405
made of a p-type InGaAs crystal; a cap layer
406
made of a p-type InAlAs crystal; and a contact layer
407
of a p-type InGaAs crystal are made to sequentially grow on a main surface of a substrate
401
through a MBE (Molecular Beam Epitaxy) method, and thereafter the contact layer
407
, the cap layer
406
, the absorption layer
405
and the field control layer
404
are etched to form a first mesa
408
on the substrate
401
.
Next, a regrown layer
409
having substantially the same height as that of the first mesa
408
, made of a compound semiconductor crystal and having a low impurity density is made to grow on the substrate
401
, and thereafter the regrown layer
409
and a crystal layer that is a lower layer thereof are etched to form a second mesa
410
around the first mesa
408
. Then, a protecting film
412
and electrodes
413
and
414
are formed on the substrate
401
and further an anti-reflection coating
415
is formed on the rear surface of the substrate
401
, and thereby the embedded mesa-type semiconductor photodetector is completed.
The embedded mesa-type semiconductor photodetector having the above-mentioned structure has an advantage of being capable of reducing a dark current in comparison with a simple mesa-type semiconductor photodetector not provided with a regrown layer
409
because an electric field intensity of a pn junction (wherein an interface between the multiplication layer
403
and the field control layer
404
is a junction surface) is weakened by the regrown layer
409
. Since mechanical strength of each chip is improved by providing, around the first mesa
408
, the regrown layer
409
having substantially the same height as that of the first mesa
408
, there is also an advantage of allowing bonding to be easily performed to a wiring substrate, or the like.
SUMMARY OF THE INVENTION
In steps of manufacturing the embedded mesa-type semiconductor photodetector described above, the first mesa is formed by laminating crystal layers made of a plurality of kinds of compound semiconductors on the substrate and by patterning these crystal layers. However, since some of these crystal layers also include compound semiconductor crystal containing Al such as InAlAs crystal (for example, a cap layer made of p-type InAlAs crystal), a stable natural oxidization film is formed on the surface of Al exposed to a sidewall of the first mesa by forming the first mesa.
By this, when the regrown layer is made to grow around the first mesa, defects or surface states are formed on the interface between the crystal containing Al and the regrown layer. Especially in the case where there are defects between the cap layer having a high impurity density and the regrown layer having a low impurity density, a current path is formed between both layers, so that the dark current in the photodetector becomes large. An increase in the dark current decreases characteristics of important receiver sensitivity, and so reliability of the photodetector is reduced and, in the case of being remarkably reducing, there are some cases of not functioning as a photodetector.
An object of the present invention is to provide a technique for reducing a dark current in a mesa-type photodetector employing compound semiconductors.
The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanied drawings.
Brief descriptions of representative ones among the inventions disclosed in the present application will be made as follows.
A semiconductor photodetector according to the present invention comprises: a pn junction formed by a compound semiconductor layer with first conductivity type formed on a semiconductor substrate and a compound semiconductor layer with second conductivity type formed on an upper portion of said compound semiconductor layer with first conductivity type; a first mesa that is formed in said compound semiconductor layer with second conductivity type and whose no bottom portion reaches said pn junction; a second mesa that is formed in a regrown layer around said first mesa, the regrown layer being made of compound semiconductor crystal surrounding said first mesa, and whose a bottom portion reaches at least said pn junction; and a groove that is provided in a vicinity of the boundary between said regrown layer and said first mesa and whose no bottom portion reaches said pn junction.
A manufacturing method of a semiconductor photodetector according to the present invention includes the following steps (a) to (e) which comprise:
(a) a step of making a compound semiconductor layer with first conductivity type grow on a semiconductor substrate, and of making a compound semiconductor layer with second conductivity type opposite to said first conductivity type, grow on an upper portion of said compound semiconductor layer with first conductivity type;
(b) a step of forming a first mesa having a predetermined shape on an upper portion of said compound semiconductor layer with second conductivity type, and of etching said compound semiconductor layer with second conductivity type located in a region not covered with said first mask, up to such a depth as not to reach an interface of said compound semiconductor layer with first conductivity type;
(c) a step of making a regrown layer made of compound semiconductor crystal grow around said first mesa;
(d) a step of forming a groove by etching a vicinity of the boundary portion between said regrown layer and said first mesa up to such a depth as to reach no interface of said compound semiconductor layer with first conductivity type; and
(e) a step of forming a second mesa in each upper portion of said first mesa and said regrown layer located therearound, and of etching said regrown layer located in a region not covered with said second mask and said compound semiconductor layer with second conductivity type located in a lower portion thereof up to such a depth as to reach at least an interface of said compound semiconductor crystal layer with first conductivity type, and of thereby forming, around said first mesa, a second mesa whose a part includes said regrown layer located in a region in which said groove is formed.
According to the above-mentioned means, by providing the groove in a vicinity of the boundary portion between a regrown layer and a first mesa, the compound semiconductor crystal containing aluminum and the regrown layer are separated in the compound semiconductor layer with second conductivity type owing to the groove, and thereby no current path is formed between both layers and so a dark current can be reduced.
FIG. 22
illustrates one example of dark current-voltage characteristics of a semiconductor photodetector provided with a regrown layer around the first mesa, wherein a symbol [A] indicates the case where the cap layer is made of InAlAs crystal, and a symbol [B&
Fujisaki Sumiko
Ito Kazuhiro
Matsuoka Yasunobu
Tanaka Shigehisa
Toyonaka Takashi
Flynn Nathan J.
Gebremariam Samuel A
Opnext Japan, Inc.
Townsend and Townsend / and Crew LLP
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